At the end of 2012, the ITU (International Telecommunications Union, a branch of the United Nations) issued Resolution 612, which allotted designated secondary radio spectral slots to coastal oceanographic radars (3-50 MHz). These radars are used in worldwide, real-time operational networks to map ocean surface currents primarily, but also to monitor sea state (wave heights); warn of approaching tsunamis; and detect/track vessels. Depending on the frequency, their signals are able to propagate beyond the visible and microwave horizon due to the conducting sea water. There are about 500 such radar systems operating today across the globe.
Before Resolution 612, these radars were licensed as “experimental”, under the ITU R4.4 designation. As such, permission to operate carried no priority, and a complaint of interference to any licensed user would result in cessation of broadcasting. The new resolution, approved by delegates after a 5-year WRC12 (World Radio Conference) allotted a few but narrow secondary licensing bands for the radars' operations, ending their “experimental” era.
The narrow spectral slots approved under Resolution 612 require that many radars share the same channel. This presents the challenge of mutual interference if they must operate at the same time and are not too distant from each other. There are many reasons why simultaneous operation (rather than “time multiplexing”, i.e., taking turns on-off) is needed, primarily for emergency applications. This includes tsunami warning, search and rescue, oil-spill management, and vessel tracking. This challenge has been solved by an invention germane to radars using frequency-modulated continuous wave (FMCW) modulation described in U.S. Pat. No. 6,856,276, the entire disclosure of which is incorporated herein by reference. FMCW modulation is used by nearly all HF oceanographic radars worldwide. It employs a slow linear sweep of frequency over a period up to one second, then continuous repeats as described in U.S. Pat. No. 5,361,072, the entire disclosure of which is incorporated herein by reference. The sweep bandwidth determines the range resolution, e.g., 50 kHz gives 3 km; a typical scenario. The invention described in U.S. Pat. No. 6,856,276, the entire disclosure of which is incorporated herein by reference, overlaps the signals from many radars but offsets their sweep start times, thereby ensuring that the information spaces of each radar do not overlap so they do not mutually interfere. This requires precise timing stability that depends on GPS signals to synchronize.
Part of the condition for authorized use of these ITU-authorized bands is the requirement that each radar must transmit a Morse-coded call sign issued by its national authority (e.g., in the U.S., this is the FCC—Federal Communications Commission). The unique call sign identifies the transmitter. The worldwide process of implementing this has begun.
As of mid-2016, no oceanographic radars operating in the MF-UHF spectral regions have begun broadcasting call signs. In response to the ITU Resolution 612, this is expected to commence in the near future, with each country demanding compliance at differing times. No guidance is given in the ITU resolution or subsequent rulings on how to achieve an acceptable call-sign format, nor must the methodologies be identical. The only requirement is that the call sign for each radar must be broadcast at least every 20 minutes; that the normal universal 6-character string (alpha-numeric) be broadcast in International Morse Code and at a rate of about 15 words per minute. The software and firmware requirements for call-sign broadcast are specific to a given brand. Hence it will be up to the commercial manufacturer to implement this for their own FMCW radar. Since over 40 countries would be users of coastal oceanographic radars, and there are fewer than four vendors, they would be required to manufacture or modify their brand radars—upon request by the owners/operators—in different countries.
Low-frequency backscatter and bistatic radar systems, operating in the MF, HF, VHF, and UHF bands, are widely used for mapping and monitoring water surface targets such as currents, vessels, and waves on the ocean, or on rivers. Nearly 150 such HF/VHF radars presently operate along the U.S. coasts as part of the U.S. Integrated Ocean Observing System (IOOS) program of the National Oceanic and Atmospheric Administration (NOAA), and such systems output their data to public U.S. websites (hfradar.ndbc.noaa.gov). Several other countries now have similar radar networks on their coasts. A total of at least 500 of these oceanographic radars are deployed and operate worldwide.
At least two backscatter radars are normally needed to map currents, because each radar measures only a scalar radial vector component, and a view from two directions is needed to construct a total 2D vector for a map. These scalar velocities are based on the Doppler principle, after separating the known Bragg-wave velocity from the unknown current velocity. In the case of a vessel target, its position and radial velocity are measured by a single radar, but a view from two radars offers the advantage of increased detection and tracking robustness.
Range or distance to the target or scattering cell is obtained from the time delay between transmit and received echoes, as is the case in all radars. After range processing, the complex (real and imaginary) echo time series for each range cell is Fourier transformed to obtain Doppler spectra and/or cross spectra among several receive antennas or elements. The velocity of the echoing target (current or vessel), as well as its bearing, is extracted from the signals at this point. One suitable and widely used bearing determination algorithm is Multiple Signal Classification (MUSIC), a direction-finding (DF) technique described in U.S. Pat. No. 5,900,834, the entire disclosure of which is incorporated herein by reference. This backscatter radar makes its measurements in a polar coordinate system in which radial current velocity at each point in the coverage area is measured by each radar on the polar grid.
In networks of coastal radars, greater data coverage and robustness for a given number of backscatter radars can be obtained by synchronizing these systems to a stable timing base and operating them multi-statically. The methodology for this is discussed in U.S. Pat. No. 6,774,837, the entire disclosure of which is incorporated herein by reference. The transmitter of one backscatter radar illuminates the sea surface, for example, where it is scattered by the waves or vessel target, and returns as echo to a different backscatter receiver. While thusly operating bistatically, each radar receiver continues simultaneously receiving echoes in its normal backscatter mode. A convenient and inexpensive multi-static synchronization method in common use employs the stable time base of GPS satellite signals; see U.S. Pat. No. 6,856,276 incorporated herein by reference above.